Ouch new they are very EXPENSIVE. Even if you had a dual carb Offenhauser intake that many of these ready to run units are made from I bet you'd still have alot into a machine shop making one for you.
https://www.blowerdriveservice.com/BM-4056P-383-CHRYS-671-871-MANIFOLD-POLISHED-2-PIECE_p_257.html

Indy sells a couple blower manifolds for B engines. They are not cheap $$$ but they are much less than that Bds.

Since you are more interested in looks than power You could do the old adapter plate on a dual 4 bbl intake.

I feel your pain.

Several years ago I was searching for one. I finally found one new but a display model, 800.00 with no return, supposed to be for a B block. Bought it but it was for a RB instead.
Part # indicated it was a B manifold. Ended up having a friend machine it to fit and been using it since.

Only reason for all the above is to let you know you could use a RB manifold as well, if you are willing to machine it.

Dave

PS. B block blower manifolds are hard to find

I haven't seen this - so I would need to get a dual quad intake first, then bolt the adapter to it?

Also, I don't want to get into a situation where I have cobbled something together and it doesn't leave clearance for the distributor

Or do you think it would be reasonable to use a set of RB to B intake adapter plates or am I creating a blowout situation?

Ouch new they are very EXPENSIVE. Even if you had a dual carb Offenhauser intake that many of these ready to run units are made from I bet you'd still have alot into a machine shop making one for you.
https://www.blowerdriveservice.com/BM-4056P-383-CHRYS-671-871-MANIFOLD-POLISHED-2-PIECE_p_257.html

Call Dyers here in greater Chicagoland. http://dyersblowers.com/ (708) 496-8100 Always had good service from them. S/F....Ken M

The 1958-62 350, 361 & 383 "B" factory 2 X 4 iron manifold is a divided plenum single plane like the Offy.



I don't think a dual plane 2 X 4 is a good choice.
I wrote some comments on my site here: http://victorylibrary.com/mopar/blower/blower-GMC.htm
Probably of little impact for a street car, but it's been 30 years since Eaton showed what's wrong with helixed roots blowers: the highest efficiency for the discharge port is not a rectangle but a much shorter triangle located directly behind the drive end. All of the manifolds I've seen have this same serious design flaw.
The $$$ 14-71 Kobelco etc. do not. The entire blower is moved backward on the engine to position the triangle midway in the intake port row.